There are lots of one-hit wonders in the app world. But there aren’t many companies that have figured out how to nurture a stable of successful apps.

Here’s one of them: Smule, which makes a series of playful music apps, like Magic Piano and Songify. The company doesn’t have a Candy Crush in its portfolio, but it has been steadily growing its audience — and revenue — for years.

Now it is getting a reward for that growth, via a $16.5 million investment led by Roth Capital Partners. Earlier investors including Bessemer Venture Partners, Shasta Ventures and Granite Ventures are back as well.

Smule’s apps use the now-standard freemium model, which means they make most of their money from a very small slice of their customers. About 200,000 of Smule’s 18 million monthly visitors pay to use its apps, starting at $2.99 per week. But there’s still real money there — the company says it is on track to do about $30 million in revenue this year.

Smule has now raised $42 million, and CEO Jeff Smith says his plan is to use the money to help expand into new markets — he figures Korea should be particularly good for his apps. He’ll also face competition there from the likes of YY, the Chinese music and games powerhouse.

Here’s a Magic Piano demo. You can also see a video of me not using Songify at our Dive into Media conference a couple of years ago over here.